Liquid metal cooling system

ABSTRACT

A method and apparatus for cooling a heated surface by means of liquid metal. A porous layer is spaced a short distance from the heated surface and the gap therebetween is filled with a liquid metal held under pressure to prevent its boiling. After the liquid metal passes through the porous layer it vaporizes, cools, is condensed and recirculated.

United States Patent 105, (Porous Digest) [72] Inventor Eleonoor Van Andel References Cited UNITED STATES PATENTS [21] 1881041 101932 B 16510 2 Filed J 10 1968 1 en arnm 5 1 Patented Man 2,197] 3,138,009 6/1964 Mccrelght 165/105 [73] Assignee European Atomic Energy Community Primary Exami er Ch-ar]e s k l (Euratom) Attorney-Stevens, Davis, Miller and Mosher Brussels, Belgium 32] Priority June 16, 1967 [33] Germany 31 P-l5 51 454.2 M

[54] LIQUID METAL COOLING SYSTEM I 3 clalms3 D'awmg ABSTRACT: A method and apparatus for cooling a heated- [52] US. Cl 162/2, surface by means of liquid metal. A porous layer is spaced a- 165/105 short distance from the heated surface and the gap [51] Int. Cl F25b 13/00 therebetween is filled with a liquid metal held under pressure Field of Search /1, 2, to prevent its boiling. After the liquid metal passes through the porous layer it vaporizes, cools, is condensed and recirculated.

LIQUID METAL COOLING SYSTEM The invention relates to a liquidinetal cooling system for nuclear reactors.

Liquid metals, more particularly alkaline metals, have recently been foundto be suitable coolants for rapid nuclear reactors, since they have satisfactory resistance to radiation, a high boiling temperature and a low moderating effect. However, their boiling state is still difficult to control, since the boiling delay as compared with that of water can be considerable, and the high coefficient of heat transfer makes boiling start explosively at the same time throughout the whole volumeof the metal. Nor hasthe problem been completely solved by the artificial seeding of boiling on structured surfaces or the like. On the one hand, therefore, for safety reasons boiling must be avoided in nuclear reactors until it has become dynamically controlled, whileon the other hand it is precisely a cooling cycle with pliaseialtemation liquid-vaporliquid which can remove such larger quantities of heat than a single-phase cycle. I

It is an object of the inventionto provide aneffective cooling system for heated walls which uses liquid metal in a cooling cycle with phase alternation without the occurrence of boiling problems.

The invention provides a method of cooling a heated surface in which a liquid metal is introduced into agap between the surface and a porous layer disposed in front of or around said surface, said liquid metal in said gap being undera pressure such that it does not boil in the gap atthe temperature to which it is heated by contact with the surface. The liquidis passed through the porous layer to azone of lower pressure where it vaporizes and the vapor then 'flows to a heat sink where it is condensed for recirculation.

The invention also provides a liquid metal cooling system for a heated wall by means of a fine porous layer which is disposed in front of the wall at a small predetermined distance therefrom, and liquid metal is introduced into the, gap produced by this distance at a pressure such that the liquid metal does not boil at the heating temperature, but passes through the pores in the layer the liquid metal to reach a zone of lower pressure, in that zonethe liquid metal vaporizes and then flows to a heat sink, forinstance a heat exchanger. Ad-

vantageously, the pressure in the gap is produced by a liquid metal reservoir disposed at a suitable height above the wall.

The principle andan embodiment of the invention will now be described with reference to the accompanying drawings,

wherein:

FIG. 1 is a partial section through a heating rod with a cooling system according to the invention;

FIG. 2 is a graph showing the radial temperature and pressure distribution corresponding to FIG. .1; and

FIG. 3 is a diagrammatic section through a complete nuclear reactor with the cooling system according to the invention.

The principle on which the invention is based can be best explained with reference to FIG; 1, which shows an internally heated rod 1 enclosed by a porous tube.2. The rod diameter and the tube internal diameter are such as to leave a uniform predetermined distance (gap 3) betweenthe two members and a liquid coolant-cg, an alkali metal-isforced into the gap 3 under pressure.

The optimum width of the gap is of the order of magnitude of up to 1 mm., so that precisely the required quantity of coolant can be introduced without excessive pressure losses. Too

wide a gap would have an adverse effect on heat transfer. The pressure on the coolant is such that boiling does not occur .with the rod at maximum temperature. As a rule a difference in pressure of 0.5 atmospheres gaugebetween. the gap and the vaporspace outside the porous tube is adequate.

The difference in pressure forces a predetermined quantity of coolant per unit of surface through the tube 2, the coolant vaporizing as soon as it emerges from the pores. Clearly,.this cooling method operates satisfactorily only with the use of metals, because of their high thermal conductivity. FIG. 2 shows the radial course of temperature: from the rod surface onwards. The temperature (continuous line) drops only a little in the gap and in the tube, so that vaporization on emergence from the pores is ensured substantially exclusively by the difference in pressure (chain line). (The jump in pressure on emergence from the pores is caused bythe high surface tension of the liquid metal and meniscus formation in a capillary channel). 1

Tests have shown that clogging of the pores can be avoided, since vaporization takes place of course only at the pore mouths. After a while any contaminations which become deposited at the pore mouths during vaporization are automatically released by the hot liquid metal which continues to flow. A cleaning circuit therefore needs to extend only over the reactor bottom to remove all foreign bodies.

' The size and density of the pores in the tube are such that at the selected liquid pressure the quantity of liquid which emerges through the pores just ensures complete vaporization. Leakage or discharge of the liquid on the vapor side, and dry- .ing out of the pores should both be prevented. With changing heat output, this requirement seems difficult to meet, but in factthis can easily bo done because of the pumping-effect of the capillary'forces, if the outersurface of the tube slightly dries out, a curved surface is produced] in each pore, so that the capillary pumping effectis increased, and more liquid material is delivered. Calculations have shown that neither leakage nor drying out is caused by a change in the heating surface loading in the ratio 1 50. Energy densities of, for instance SOOW/Cm and 10 W/Cm can therefore be reliably removed by the same cooling arrangement; The cooling system is therefore very stable-a great advantage in view of the safety requirements of nuclear reactors.

FIG. 3 shows diagrammatically in simplified form a reactor core whose rod-shaped fuel elements 4 are inserted in porous tubes 5, for instance of sintered steel, as shown in FIG. 1. A reservoir 6 for the liquid metal is disposed above the reactor core and communicates with the gaps in all the fuel elements. The pressure is produced by a pump which ensures that the level of the liquid material in the reservoir 6 remains constant. As shown in FIG. 3, the pump can act on a pump chamber 8 below the fuel elements via a feed line 7.

The reservoir 6 both maintains pressure and also provides a I very effective biological screen in the upward direction and pump to the pump chamber 8. The heat exchangers are preferably disposed in a circle directly at the core edge, so that the vaporcan flow out in all directions through and between the pipes 5. The speed of the vapor can be the speed of sound; this is a adiabatic expansion with a large difference in pressure.

.The high transportation of heat energy and the relatively small amount of coolant in the reactor core in the system according to the invention enable a compact rapid neutron reactor to be designed with a high breeding ratexHowever, the system according to the invention can advantageously also be used in thermal reactors.

I claim:

.l. A method of cooling a heated surface comprising:

a..introducing a liquid metal into a gap between the surface and a porous layer disposed in front of said surface;

b. pressurizing theliquid metal in the gap so that it does not boil in the gap when heated by contact with the surface; c. passing the liquidthrough the porous layer to a zone of lower pressure where it vaporizes and the vapor then flows to a heat sink where it is condensedfor recirculation.

e. a zone of lower pressure outside the porous layer so that said liquid metal can vaporize after passing through the porous layer;

f. a heat sink in said zone of lower pressure to condense the vaporized liquid metal.

3. A liquid metal cooling system as claimed in claim 2, in

which the pressure in the gap is produced by a liquid metal reservoir disposed at a suitable height above the surface. 

2. A liquid metal cooling system for a heated wall comprising: a. a wall enclosing a source of heat; b. a fine porous layer disposed in front of the wall at a small predetermined distance therefrom to leave a gap therebetween; c. a liquid metal in said gap; d. means to maintain said liquid metal under pressure so that it does not boil at the heating temperature; e. a zone of lower pressure outside the porous layer so that said liquid metal can vaporize after passing through the porous layer; f. a heat sink in said zone of lower pressure to condense the vaporized liquid metal.
 3. A liquid metal cooling system as claimed in claim 2, in which the pressure in the gap is produced by a liquid metal reservoir disposed at a suitable height above the surface. 